1. Field of the Invention
The present invention relates to a system and method for controlling the cruising speed of a vehicle, particularly to a system and method which achieves good response characteristics to changes in the inter-vehicle distance between the controlled vehicle and another preceding vehicle.
2. Description of the Prior Art
Japanese Patent Application Open No. 55-86000 discloses a system for controlling the vehicle speed at a constant predetermined speed value and at a speed value which is dependent upon the detected distance between the controlled vehicle and a preceding vehicle so that the controlled vehicle can automatically follow the other vehicle.
In the above-identified reference, the disclosed system controls the vehicle speed as described below with reference to FIG. 1. When the inter-vehicle distance D between the vehicle and the preceding vehicle is equal to or greater than a safe inter-vehicle distance value Do related to the set vehicle speed So to which a constant distance value (, e.g., 1 meter) is added (, i.e., Do+1, Do-1 in FIG. 1) used in cruise control operation, the system maintains the vehicle speed if the vehicle speed S is within a preset speed range (for example, 0.5 km/h) with respect to the set vehicle speed So. However, when the vehicle speed S exceeds (So+0.5)km/h as the vehicle rolls down a slope, the system actuates a throttle valve to throttle down by an angle, i.e., deceleration rate .DELTA..theta. proportional to the speed differential .DELTA.S (for example, 3.DELTA.S) in order to return the vehicle speed to the set speed So. Conversely, when the vehicle speed S drops below (So-0.5)km/h as the vehicle ascends a slope, the system opens the throttle valve through an angle which represents a desired acceleration rate .DELTA..theta. and which is proportional to the speed differential .DELTA.S (for example 3.DELTA.S) in order to accelerate the vehicle toward the set speed So. When the vehicle speed S is below (So-5)km/h, the system accelerates the vehicle at a constant acceleration rate .DELTA..theta. (for example 2.degree.).
On the other hand, when the measured inter-vehicle distance is more than about a meter below the safe inter-vehicle distance value Do, the system decelerates the vehicle at a constant deceleration rate .DELTA..theta. (for example 3.degree.). Since the safe inter-vehicle distance Do decreases as the vehicle speed S is reduced by the cruise control system, the vehicle will soon be trailing the preceding vehicle at a safe distance and at approximately the same vehicle speed. When the inter-vehicle distance D between the two vehicles is within a preselected range of the safe inter-vehicle distance Do hereinafter referred to as a dead zone, speed control is not carried out.
In the above vehicle cruising control system the vehicle speed is increased or decreased in accordance with a safe inter-vehicle distance value Do derived solely on the basis of the vehicle speed. That is to say, the conventional system performs the control over the vehicle speed merely according to the difference between the periodically-measured inter-vehicle distance D and the safe inter-vehicle distance as shown in FIG. 1 without taking the vehicle speed of the preceding vehicle into consideration. Therefore, in cases where the vehicle speed of the preceding vehicle is significantly lower than that of the trailing, controlled vehicle which is therefore abruptly approaching the preceding vehicle, the time at which the cruise control system begins to decelerate the vehicle is so late that the driver needs to apply the brakes himself. On the other hand, in cases where the vehicle is following another vehicle at a constant safe inter-vehicle distance Do and the other vehicle abruptly accelerates so that it quickly moves away from the vehicle, the controlled vehicle will not promptly begin to accelerate so that the inter-vehicle distance will become greater than necessary; in other words, the vehicle will not suitably accommodate traffic flow.